1. Field of the Invention
The present invention relates to a liquid crystal display element and a projection type display device using the liquid crystal display element, and particularly relates to a liquid crystal display element using twisted nematic type liquid crystal used for a liquid crystal layer and a projection type display device using the liquid crystal display element.
2. Description of the Related Art
In a liquid crystal projector and other projection type display device, a light emitted from a light source is divided to red, green and blue, the respective color lights are modulated by three light valves composed of a liquid crystal display element (hereinafter, referred to as an LCD), color light fluxes after the modulation are again composed and projected by being enlarged on a projection surface. As a light valve for a liquid crystal projector, etc., an active matrix drive type LCD driven by a thin film transistor (hereinafter, referred to as TFT) is generally used.
A nematic liquid crystal is used in most of active matrix drive type LCDs and an optical rotatory mode LCD may be mentioned as the main display method. An optical rotatory mode LCD is a twisted nematic (TN type) liquid crystal having molecular arrangement twisted by 90 degrees, which is, in principle, black and white display and has a high contrast ratio and preferable gray scale display capability.
To attain uniform display of an active matrix drive type LCD, it is necessary that liquid crystal molecules are uniformly aligned allover a substrate surface.
In the substrate formed with two electrodes formed with an alignment film, the alignment films of each substrate are arranged to face to each other and put together with a seal on a seal region around a pixel display region on which an image is actually displayed. To control a space between the substrates, a globular spacer called a micropearl or a columnar spacer formed by a resist is used before putting together as explained above.
Vacant cells are formed through the process, then, liquid crystal is encapsulated in the vacant cells, so that liquid crystal cells are produced.
Note that the above liquid crystal is composed of several kinds of elemental materials of a liquid crystal and also is called a liquid crystal composition. A light refractive plate is attached to the produced liquid crystal cell and a liquid crystal display element is produced.
When applying a voltage to pixel electrodes arranged in matrix, a line inversion drive method for inverting the application voltage on every other line and a column inversion drive method for inverting the application voltage on every other column are widely used to improve display quality.
Since the application voltage is inverted between adjacent pixel electrodes in these driving methods, as shown in FIG. 14, it suffers from the disadvantage that a reverse tilt domain RTDM having an opposite tilt direction from a pre-tilt direction originally given by the liquid crystal is generated in a pixel portion corresponding to each pixel electrode PXLE (e.g. Japanese Patent No. 2934875).
Particularly, in a normally white mode liquid crystal display, void occurs on a disclination line DSCL on a boundary with a normal region and contrast declines. Therefore, a tilt angle is made large to make the reverse tilt domain RTDM small, but a disadvantage of deteriorating production yield is brought. Also, leaking of a light is reduced by providing a light block material in accordance with a position where the reverse tilt domain arises, but there is a disadvantage that the aperture ratio is reduced.
Thus, as one of methods to improve the contrast ratio in the TN type LCD, a method of heightening an effective voltage to be applied between a common electrode and pixel electrode facing to each other, that is, widening a dynamic range has attracted attention.
When heightening the effective voltage to be applied between the common electrode and pixel electrode facing to each other as above, not only that alignment of liquid crystal molecules becomes more vertical, but the position where the disclination line DSCL arises shifts to the peripheral direction (outer side) in the pixel portion, so that the contrast ratio can be improved.
Also, an active matrix drive type LCD used as a light valve of a projection type display device is made more compact as a liquid projector and other projection type display device gets more compact, while pixels are pursued to be finer and brighter. As the pixels become finer, pitch intervals of pixels of liquid crystal display element becomes narrower. For example, in the case of a type where a substrate size is 22.9 mm (0.9 inch) extended graphics array (XGA), the pixel number becomes 1024×768 and the pixel pitch becomes 18 μm.
However, as pixels gets finer, spaces between transparent electrodes of each pixel (for example, ITO: indium tin oxide) become furthermore narrower and an electric field in the crossing direction is generated when a potential in each pixel is inverted. Due to the electric field in the crossing direction, disadvantages arise that an alignment of liquid crystal molecules on a transparent electrode boundary portion is distorted, a boundary between the distorted portion and normal portion (also referred to as a disclination line) arises as a display defect, and the contrast declines. Furthermore, as pixels get brighter, the display defect tends to be more notable.
To solve the disadvantage, a method of changing a position of the above boundary line in accordance with a position of a contact hole has proposed (for example, refer to the Patent Article 2). Also, a method of controlling the electric field in the crossing direction by making a TFT to have different levels has proposed (for example, refer to the Patent Article 3).
[Patent Article] The Patent Publication No. 2934875 paragraphs 0005 to 0006
[Patent Article] The Unexamined Patent Publication No. 2001-265255 (pp. 8 to 13, FIG. 2 and FIG. 3)
[Patent Article] The Unexamined Patent Publication No. 2001-265255 (p. 19, FIG. 17)
However, when the effective voltage is made higher as explained above, a reverse tilt domain disappears in a part of pixels and the disappearance state becomes a quasi-stable state. Thus, as shown in FIG. 15, hysteresis arises in V (voltage)−T (light transmittance intensity) characteristics, and when changing from black display to intermediate grade display, an image quality defect by an extinction point and, particularly, a serious image quality defect by an extinction line as a result that a display defect of the reverse tilt domain propagates to an adjacent reverse tilt domain are caused.
The above image defect will be referred to as a stripe domain.
As a result, it is impossible to apply a voltage (effective voltage) of a certain value or more to between the common electrode and pixel electrode facing to each other, and there is a disadvantage that a desired contrast ratio cannot be realized. This disadvantage notably arises when a distance between adjacent pixel electrodes is made short to realize a finer liquid crystal display element with a high aperture ratio.
Accordingly, in a matrix type liquid crystal display device comprising a switching element, such as a thin film transistor (TFT), it is difficult to strike a balance between the high aperture ratio and the high contrast ratio.
However, both of the inventions described in the Patent Articles 2 and 3 are a method of suppressing an electric field in the crossing direction by changing a shape to strengthen an electric field in the vertical direction.
Therefore, there are disadvantages that (1) due to the provision of different levels, there arises a region where rubbing cannot be sufficiently performed and a display defect is caused, and (2) due to the complex configuration, the yield declines and the production cost becomes high.
Also, as the most effective countermeasure, there is a method of preventing an affect by the electric field in the crossing direction by strengthening the electric field in the vertical direction of the each substrate by making a cell gap thinner. In the case where two light refractive plates are orthogonally arranged and the display is normally white display being white when not turned on, it is designed to be capable of obtaining the maximum transmissivity when a refractive index anisotropy Δn of the liquid crystal material×cell gap=480 nm from the gooch-tarry formula. Namely, to obtain the maximum trasmissivity, the refractive index anisotropy Δn of the liquid crystal has to be high when applying the countermeasure of making the cell gap thin as explained above.
In the cell configuration of making the cell gap thin as above, space controlling by a columnar spacer having been applied in recent years is essential, but there is a disadvantage that distortion of a stripe domain arises around the spacer portion due to formation of the spacer. Also, as those applied with a liquid crystal material having a high Δn, in pixel pitch of 18 μm and a substrate size of 22.9 mm (0.9 inch) XGA, a liquid crystal material having a Δn of less than 0.16 is currently used and the cell gap is 3.0 μm. In the above cell design, a stripe domain arises at a voltage of 0.4 V or less, which is lower than a normal drive voltage of 5.0V, and ends up in a display defect.